Abstract: A hot fluidized bed reactor has been widely used in sustainable agriculture in recent years. Complex A complex flow of gas-solid reaction can be found in the internal reactor, including the distribution of gas components, gas velocity, particle composition, particle velocity, pressure distribution, and temperature distribution within the bed. However, these parameters are often difficult or impossible to measure during the actual operation of the reactor, due mainly to the experimental conditions and conventional measurement. Numerical simulation can be expected to identify the operating parameters in the fluidized bed reactor, in order to optimize the reactor design and operation. Furthermore, the iterative promotion of the reactor can be accelerated to significantly reduce the design and construction costs. In this study, the systematic simulation was carried out on a 10 t/d sludge and biomass co-incineration fluidized bed reactor using the Computational Particle Fluid Dynamics (CPFD). The fluidization and reaction states of the reactor were determined under both closed and open secondary air conditions. The simulation results indicate that the reactor operated normally under rated design parameters, with the reasonable gas-solid fluidization and high fuel conversion rates, particularly with the minimal fuel slip in the flue gas. In the fluidization state, the bed material reached the stable fluidization within 2 s, primarily concentrated in the lower part of the reactor, when the secondary air was closed. The concentration of particles in the dilute phase decreased with the height, with no significant particle outflow observed. Furthermore, the reactor exhibited a core-annulus structure during stable operation, with the solid content in the center lower than that at the wall. The highest pressure was found at the air distribution plate. The significant fluctuations of pressure occurred at various measurement points after fuel addition, indicating the intense combustion disturbances. The distribution of gas-solid velocity showed that the high gas velocity was disturbed in the lower region, while the slower was disturbed in the upper region, leading to the vigorous fluidization of the bed material. Once the secondary air was opened, the total air volume remained constant, resulting in a decrease in the primary air while with an increase in the secondary air. The height of the dense phase region was reduced to an increase in the particle concentration, indicating a bubbling fluidization state. The airflow velocity and disturbance level significantly decreased, while the disturbances near the upper secondary air inlet increased, indicating a counterflow state. In the reaction state of the reactor, the fuel was rapidly realized the pyrolysis and combustion, with the char burning out rapidly, and then the ash was concentrated in the lower bed layer, when the secondary air was closed. Gaseous fuels (such as CH₄, C₂H₄, CO, H₂) primarily combusted above the feed inlet, resulting in the high concentrations of CO₂ and H₂O, with O₂ nearly depleted. Nitrogen pollutants (HCN, CNO, NO) were mainly converted in the middle and lower sections, while the sulfur pollutants (H₂S, SO₂) were distributed in the upper part. The high-temperature zone was expanded in the upper section, reaching the maximum temperature of 1217 K. A pilot test was carried out to verify the simulation. The maximum temperature in the furnace during the test was 1163.15K with an error of 4.6%, which was within the acceptable range. There was also the a uniform distribution of temperature in the bottom dense phase region, indicating the high gas-solid convective heat transfer rate. After opening the secondary air, the oxygen supply from the primary air decreased, leading to an increase in the gaseous fuel concentration and an expanded distribution area, and limited to below the secondary air flow path. The disturbances and oxygen supply from the secondary air was promoted the rapid fuel burnout, resulting in the more uniform distribution of pollutants CNO and NO, while high concentrations of SO₂ were concentrated at the top of the reactor. These research findings can provide the guidance to explore the fluidization and reaction states in the sludge and biomass co-incineration fluidized bed reactors, particularly for the structural design and optimization of reactors.